The invention relates to an optical arrangement for a microscope, with a main beam path in which a microscope objective and an observation tube are located, and with an additional supplementary beam path which can be selectively interposed between the microscope objective and the observation tube in place of a portion of the main beam path, and an optical deflection element which can be inserted into the main beam path and withdrawn from the main beam path to select between the main beam path and the supplementary beam path, wherein the supplementary beam path includes at least first, second and third optical surfaces each deflecting the beam, the third optical surface deflecting the supplementary beam path into the observation tube.
An example of an optical arrangement of this type is given in Austrian Patent Specification 333,052. In that Specification, the supplementary beam path is provided when the optical deflection element is removed from the optical main beam path, whilst when the optical deflection element is inserted into the optical main beam path, the beam coming from the microscope objective is deflected directly into the observation tube. Consequently, the supplementary beam path is aligned with the optical axis of the microscope objective as far as the first optical surface deflecting the beam, and until then runs generally vertically. The three optical surfaces deflecting the beam then deflect the supplementary beam path in such a way that it ultimately coincides with the main beam path entering the observation tube. According to Austrian Patent Specification No. 333,052 or the corresponding U.S. Pat. No. 3,924,926, at least one image of the object field or at least one image of the object field and an image of the exit pupil of the objective will be located within the additional beam path before the beam is first deflected, and furthermore at least one element controlling polarisation will be provided near one of these images. The advantage of this arrangement is that all optical elements interfering with polarisation are avoided.
Polarisation microscopy makes a basic distinction between two different observation methods, orthoscopic and conoscopic observation. In orthoscopic observation, among other things the shape, size distribution, colour structure and grain boundaries of a thin section of rock or stratum are examined in polarised light, whereas in the conoscopic observation of a crystal grain discovered orthoscopically the interference phenomenon arising in the objective exit pupil is observed. This so-called axis interference makes it possible to determine the number and angles of the axes and other optical properties of a crystal. To display this axis interference, a thin section of a particle of interest is first examined in polarised light. A focusable auxiliary lens, the Bertrand lens, is then placed in the beam path, focussed on the exit pupil of the high-aperture objective used and centered, if necessary, the particle is isolated by means of a pinhole diaphragm and the interference pattern obtained is observed.
The known Reichert microscope sold under the trademark Zetopan Pol has in the monocular observation tube a focusable Bertrand lens which is located at the optically correct point and which can be moved into and out of the beam path. It is therefore possible in this instance to change quickly from orthoscopic observation to conoscopic observation. However, the length and circumference of the monocular observation tube are considerable. But above all, only monocular and not binocular observation is possible.
In the known Leitz microscope sold under the trademark Orthoplan Pol, which does have a binocular observation tube, a Bertrand lens and a diaphragm can be moved into and out of the main beam path of the microscope before deflection into the inclined binocular observation tube. However, a compromise is obviously reached here as regards the optical positions of the Bertrand lens and of the diaphragm, and the overall height of the microscope is also considerable.